The present invention relates to a plasma processing apparatus and method; and, more particularly, the invention relates to a plasma processing apparatus and method which employ a high-frequency power supply for performing plasma processing on a sample, such as a semiconductor wafer.
Known plasma processing apparatuses, such as those disclosed in Japanese Patent Publication No. 4-69415 and Japanese Patent Laid-Open No. 2001-85395, modulate microwave power or high-frequency power that has been applied to an electrode so as to control the proportion of ions or radicals in a plasma for the purpose of enhancing the accuracy of etching of a semiconductor wafer. These publications also describe techniques that are capable of controlling ion energy so as to carry out high-precision etching.
Etching apparatuses using a plasma to process a semiconductor water are widely used. One example is an etching apparatus employing an ECR (electron cyclotron resonance) system. FIG. 6 shows an example of such a system. In this system, a DC current is passed through a solenoid coil disposed outside of a chamber 1 so as to produce a magnetic field within the chamber. Furthermore, a high voltage is applied to an oscillator 3 so as to generate, for example, a microwave, which is then introduced into the chamber. An electron cyclotron resonance (ECR) occurs due to the synergistic effect of the microwave and the magnetic field, turning a process gas 4, that has been introduced into the chamber, into an ECR plasma 5. High-frequency power is applied to an electrode 6 by a high-frequency bias power supply 7 so as to attract ions that are included in the plasma in a direction perpendicular to the surface of a semiconductor wafer 8. In addition, the incident ion energy is independently controlled so as to carry out anisotropic etching with high precision.
Generally, each of the many parameters used for performing an etching process is set to a certain value or mode beforehand. These parameters include the processing time, the pressure within the vacuum chamber, the intensity of the current that passes through the solenoid coil, the intensity of the microwave power, the amount of the introduced process gas, and the intensity of the high-frequency power that is applied to the electrode. The control microcomputer included in the etching apparatus simultaneously controls each component device that is necessary to perform the required processing. Furthermore, if the etching process includes a plurality of process steps, the control microcomputer controls the value or the mode for each component device based on the parameter setting for the next process step, each time process steps are switched. A series of parameter settings for an etching process made up of a single or a plurality of process steps is hereinafter referred to as “a recipe”.
A description will be given of the modulation of the output of the high-frequency bias power supply 7. To carry out the desired processing with high precision, the output typically is turned on for only a certain period (indicated by a duty ratio) in a given interval and is turned off for the remaining period in the interval, and this operation may be repeated, for example. In this case, the actual output power of the high-frequency bias power supply 7 is equal to the product of the peak value of the high-frequency power (hereinafter referred to as “the output intensity”) and the duty ratio. It is known that a strong correlation exists between this actual output power and the etching speed. Therefore, to realize the same etching speed as that obtained with the continuous mode (in which the output is not modulated), a modulation with a duty ratio of 10% requires an output intensity approximately 10 times as high as that for the continuous mode, so as to obtain the actual output power of the continuous mode.
Furthermore, the entire etching process may be made up of individual process steps in which the high-frequency power applied to the electrode is modulated (the modulation output mode) and those in which the high-frequency power is continuously output (the continuous output mode) in combination. For example, the process steps for main etching may be carried out in the modulation output mode, while the process steps (overetching process steps) for removing portions unintentionally left unetched are carried out in the continuous output mode.
However, when a performance improvement utilizing such a modulation mode output function was studied, occurrences of unexpected wafer damage were encountered. The investigation by the inventors of the cause of the damage has revealed that use of a recipe including certain process steps may cause such damage, for the following reasons.
In particular, it was found that a problem arose when a main etching process step was switched to an overetching process step. Specifically, in actual practice, the timings of the output mode switching and the output intensity switching in the high-frequency power supply 7 do not always fully coincide with each other (even though they are required to coincide). The inventors found that, in some cases, the output mode switching tended to occur before the output intensity switching. FIG. 7 shows an example of this phenomenon. In this case, in the continuous output mode, a magnitude of power 10 times as large as a preset power value (that is, the output intensity set for the modulation mode at the previous process step) is applied continuously to the semiconductor wafer 8 for the time period corresponding to the timing difference (the larger the timing difference, the more significant the damage to the semiconductor wafer 8). The switching timing difference is caused by the time difference between the issuance of the output mode switching instruction and the output intensity switching instruction sent from the control microcomputer to the high-frequency power supply 7, by variations in the delay time of the relays and the electronic components in the power supply, etc.
The present invention has been devised in view of the above problem. It is, therefore, an object of the present invention to provide a plasma processing apparatus and method that are capable of carrying out a processing with high precision, without damaging the semiconductor wafer.